Mental and other neurological disorders frequently surface during or near the end of development and, like the changes that occur in development, their symptoms are typically permanent. This provides strong evidence that developmental pathologies may be related to incidence of these disorders. In order to understand the effects of abnormalities in development, how they may give rise to symptoms of a mental or neurological disorder, and therefore what therapies might be appropriate, a baseline understanding of the principles that govern normal development is essential. Development is physically manifested in parts of the nervous system as a rewiring of synapses between classes of neurons and their targets. For the most part, the knowledge of the mechanisms that guide this process is poor. In easily accessible parts of the peripheral nervous system like the neuromuscular junction, techniques like in vivo fluorescence imaging and electrophysiology have revealed some information about the rewiring process that occurs there: in general, classes of neurons are initially highly interconnected and over development many of the connections are pruned, while surviving synapses are strengthened, resulting in a refined neural wiring. This process is called synapse elimination and is thought to be driven by synaptic activity and therefore experience. Similar techniques have been much less informative in the central nervous system, however. In order to overcome this barrier to understanding synaptic rewiring in higher learning centers, this project proposes to use serial section scanning electron microscopy to produce 3D volumes of high-resolution images of wild-type cerebellum tissue from mice in early postnatal development. Unlike other methods, the resolution of electron microscopy is sufficient to clearly identify all synapses in a tissue sample. Serial section scanning electron microscopy, a recent adaptation of this technique in which a long series of thin sections is cut, collected on tape, and automatically imaged, is capable of imaging blocks of tissue 100s of microns thick with minimal loss, and reasonably quickly. The cerebellum is a good system to investigate because it is intrinsically simple and compact. [In this project Purkinje cells and their climbing fiber inputs wll be reconstructed and the numbers, positions, and strengths of their synapses will be measured to establish a ground truth for this information. This will then be used to provide initial insight about the mechanisms underlying cerebellar rewiring by determining, first, whether this process is a minor refinement or a major rewiring; and second, whether or not the mediators of cerebellar rewiring are similar to those of peripheral synapse elimination. This work is an essential step toward understanding the underpinnings of normal development in the central nervous system, which will eventually lay the groundwork for investigations into the underlying causes of developmental disorders. A didactic neuroscience component is present in this training for the benefit of the investigator.]